Brain size of human ancestors evolved gradually over three million years, study finds

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The average brain size of humans has increased gradually over the past three million years, a new study involving a University of Stirling researcher has found.

Analysis of 94 hominin fossils has shed light on how and when modern human brains increased to three times the size of our closest living relatives: chimpanzees and bonobos.

Experts demonstrated the trend was primarily caused by the evolution of larger brains within populations of individual species, but the introduction of new, larger-brained species and extinction of smaller-brained ones also played a part.

The research, published in the Proceedings of the Royal Society B, involved Dr Elizabeth Renner, a Research Assistant in Psychology at Stirling.

The project originated while Dr Renner, and her fellow co-authors, were working towards a PhD at George Washington University. The students were challenged to characterise how hominin brain size changed over time.

Lead author, Andrew Du, a postdoctoral scholar at the University of Chicago, said: “Brain size is one of the most obvious human traits that makes us human. It’s related to cultural complexity, language, tool making and all these other things that make us unique.

“The earliest hominins had brain sizes like chimpanzees, and they have increased dramatically since then. So, it’s important to understand how we got here.”

Dr Renner helped conceptualise the project in its early phase and collect previously published data on hominin endocranial volume and the ages of various fossils.

Dr Du and his team compared data on the skull volumes of 94 fossil specimens from 13 different species, beginning with the earliest unambiguous human ancestors, Australopithecus, from 3.2 million years ago to pre-modern species, including Homo erectus, from 500,000 years ago when brain size began to overlap with that of modern-day humans.

They observed that, when the species were counted at the clade level – or groups descending from a common ancestor – the average brain size increased gradually over three million years. Studied more closely, the increase was driven by three different factors, primarily evolution of larger brain sizes within individual species populations, but also by the addition of new, larger-brained species and extinction of smaller-brained ones.

The team also found that the rate of brain size evolution within hominin lineages was much slower than how it operates today.

The research, supported by the National Science Foundation, is entitled Pattern and process in hominin brain size evolution are scale dependent, and is available to read here.

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